How to flatten hierarchical data with SelectMany?

Short answer: How to flatten hierarchical data with SelectMany?

Explain a bit more

var departmentsWithEmployees = new[] new { Department = "IT", Employees = new [] {"Bob", "Charlie"} new { Department = "HR", Employees = new [] {"Alice", "Eva"} } var allEmployees = departmentsWithEmployees .SelectMany(d => d.Employees); foreach (var emp in allEmployees) Console.WriteLine(emp); Explanation: SelectMany flattens nested collections into a single sequence.

Real-world example (ShopNest)

ShopNest maps entities to DTOs with .Select(o => new OrderSummaryDto { Id = o.Id, Total = o.Total }) so the API does not leak internal fields.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.